1. Field of the Invention
The present invention relates to a heat accumulator, and more particularly to a heat accumulator which destroys a supercooled state of a heat storage material consisting of a liquid exhibiting a supercooled state, by applying a voltage to the heat storage material, and is capable of evolving latent heat from the heat storage material.
2. Description of the Prior Art
A heat accumulator which uses a liquid exhibiting a supercooled state such as sodium acetate trihydrate as a heat storage material, destroys the supercooled state by applying a voltage through electrodes, and evolves latent heat stored in the heat storage material is known (for example, see Japanese Unexamined Utility Model Publication No. 5-96769). In such a heat accumulator, the heat storage material receives heat from a high temperature medium and stores the heat. When its contact with the high temperature medium is inhibited or it is brought into contact with a low temperature medium, the temperature of the heat storage material drops gradually. Even when the temperature of the heat storage material falls below its freezing point, for example about 58.degree. C. in the case of the sodium acetate trihydrate, the heat storage material does not solidify, that is, enters a supercooled state. In this manner, the latent heat is accumulated in the heat storage material.
It is known, on the other hand, that solidification of the heat storage material can be promoted by destroying the supercooled state of the heat storage material by generating nuclei in the heat storage material in a supercooled state. Therefore, in the heat accumulator described above, the seed nuclei are aggregated by applying a voltage so as to destroy the supercooled state of the heat storage material, and solidification of the heat storage material is promoted by generating the nuclei in the heat storage material so as to thereby evolve the latent heat stored in the heat storage material. Heat evolution can thus be attained quickly by applying the voltage at any point of time while the heat storage material remains in a supercooled state.
When the nuclei are generated in the heat storage material by applying the voltage to the heat storage material in a supercooled state as in the heat accumulator described above, the nucleation in the heat storage material occurs on the anode surface. If the sodium acetate trihydrate is used as the heat storage material as in the heat accumulator described above, on the other hand, the heat storage material in the liquid phase is alkaline. Therefore, when the voltage is applied to this heat storage material, hydroxide ions (OH.sup.-) in the heat storage material migrate to the anode. Then, an oxide layer is formed on the anode surface due to the electrochemical reaction, and this oxide layer is laminated on the anode surface to form a plurality of layers after applying the voltage to the heat storage material repeatedly. As a result, the seed nuclei disappear on the anode surface, and the nucleation function on the anode surface gradually decreases.
To solve this problem of the decrease of the nucleation function as the result of the formation of the oxide layers, the above prior art reference proposes to fit a supplementary member comprising a screw and a nut to the anode, to restrict the oxidation reaction by these members and to improve durability. However, even in such an accumulator, the oxide is deposited in the gap between the electrode and the screw and nut, and this accumulator is not yet entirely satisfactory.
Regarding heat accumulators applied to automobiles, the temperature of the engine cooling water is generally 95.degree. C. and sometimes exceeds 100.degree. C. at the time of a high engine load, such as driving on an upward slope, high speed driving and the like. In such a case, the prior electrode for nucleation loses its nucleation function, and nucleation does not occur even if the voltage is applied to the supercooled state. This is because the seed nuclei (clusters) for nucleation retained on the electrode surface become completely molten at a high temperature. This disappearance of seed nuclei at high temperatures has not been pointed out at all in the past.
The purpose of the present invention is to provide a heat accumulator which has a high degree of durability and is capable of withstanding high temperatures.